This invention relates to a seal ring made of a synthetic resin for preventing leakage of a liquid such as oil or water and a gas such as air, and more particularly an oil seal ring made of a synthetic resin and mainly used to seal hydraulic oil in a torque converter, a hydraulic clutch and an automatic transmission in an automobile.
Conventional seal rings of this type include a metal seal ring and a seal ring made of tetrafluoroethylene resin. Now, it is considered to form oil seals by injection-molding synthetic resins to improve wear resistance and oil sealing ability and reduce manufacturing costs.
Such a seal ring is split apart at one circumferential point for the convenience of mounting. Such split ends are in the form of straight-cut type abutments 71, 71' as shown in FIG. 23 or composite step-cut abutments 72, 72' as shown in FIG. 24.
A seal ring of this type is mounted in a space between a piston and a cylinder so as to be rotatable when the piston and the cylinder rotate relative to each other and to be slidable relative to the piston and the cylinder when they move axially relative to each other. It is required that such a seal ring not damage the sliding surfaces of the mating members and have a high ability to seal a gap between the piston and the cylinder.
As shown in FIG. 25A, a seal ring 81 is ordinarily mounted in a peripheral groove 83 formed in a shaft 82 with its outer sealing surface 90 pressed against the inner periphery of a cylinder 84 to seal the space between the shaft 82 and the cylinder 84. Also, by pressing one of its side sealing surfaces 85 against a side wall of the peripheral groove 83, the peripheral groove 83 is also sealed.
For a device in which the shaft 82 is rotated, it is proposed to use one of seal rings 81 as shown in FIGS. 26-28 to reduce the sliding frictional resistance and wear produced between the sealing surfaces 85 and the inner walls of the peripheral groove 83 (as disclosed in Unexamined Japanese Utility Model Publication 3-88062).
These seal rings 81 have lubricant grooves 86, 87 and 88 in their respective sealing surfaces 85. By introducing lubricant into these grooves, a lubricant film is formed on the sealing surfaces 85.
To form such a seal ring, it is injection-molded, for ease of releasing, with its abutments 72 and 72' spaced from each other (with no radial overlapping) so that it has a substantially circular shape or a slightly spread circular shape. The seal ring thus injection-molded is mounted on a piston with its abutments 72 and 72' fitted together. The piston is inserted into a cylinder in this state. When the abutments 72 and 72' are fitted together, their corners a and b', a' and b or c and c' may interfere with each other, making it difficult to fit the abutments together.
Also, as shown in FIG. 30, an injection-molded seal ring 73 does not necessarily have a completely circular shape. That is, the abutments 72 and 72' may protrude from the outer periphery of the seal ring 73. Such protrusions will make it difficult to insert the piston 75 into the cylinder 74. Also, such protrusions will form a space 76 between the outer periphery of the seal ring 73 and the inner periphery of the cylinder. A perfect seal is thus not possible.
When the seal ring 73 is rotated in this state, it will slide with its abutments 72, 72' coming locally into contact with the mating member at their tips. This increases the contact surface pressure to such a high level that the oil film eventually disappears, causing wear on the inner periphery of the cylinder 74.
The same problem will occur with a seal ring having a straight-cut abutments 71, 71'. In the case of a seal ring with composite straight-cut abutments 72, 72', if a protrusion 77 of one of the abutments 72 and 72' has a greater thickness than the depth of a shoulder 78 of the other abutment 72', the protrusion 77 will protrude from the outer periphery of the ring, causing the abovementioned problem.
One way to solve this problem is to form a seal ring with such a curvature that it will have a completely circular shape when fitted in the cylinder 74. But it is extremely difficult to form such a seal ring because, for this purpose, many factors including contraction coefficient of the material, machining tolerance of the mold used, etc. have to be taken into account. That is, it is difficult to completely prevent local contact of the abutments with the mating surface at the tips of the abutments.
Another way to insert the seal ring into the cylinder so that it has a completely circular shape is to provide a seal ring 73 injection-molded into a completely circular shape with abutments 72, 72' by machining. But even in this case, if the curvatures of the seal ring and the cylinder 73 are not coincident due to dimensional errors in the outer periphery of the seal ring 73 or the inner periphery of the cylinder 74, it is difficult to completely prevent local contact between the abutments 72, 72' and the cylinder.
In order to maintain a high level of sealability of the sealing surface 85, any of the lubricant grooves 86, 87 and 88 is open only at the inner peripheral side, and closed at the outer peripheral side. Thus, at the outer peripheral side, the sealing surface comes into contact with the inner surface of the peripheral wall 83 over the entire circumference, so that no lubricant film can be formed therebetween on the outer peripheral side. As a result, if the shaft 82 is made of a soft material such as an aluminum alloy, the sealing surface 85 can be worn easily by relative rotation between the seal ring 81 and the shaft 82. If the shaft 82 is made of a less wear-resistant material such as aluminum, the inner surface of the peripheral wall 83 tends to wear.
A first object of the present invention is to provide a seal ring having abutments that are less likely to locally contact with the mating member, and a method of manufacturing such a seal ring by injection molding.
A second object of this invention is to provide a seal ring which is high in its sealing ability and which can form a lubricant film over the entire area of its sealing surface to improve its own wear resistance and the wear resistance of the mating member.
Some of conventional seal rings have a square or rectangular section. To form such a seal ring 81 from a synthetic resin, as shown in FIG. 25B, mating surfaces 93 of a mold 92 are positioned at the corner between one of the side faces 90 and outer surface 91 of the seal ring 81 (i.e. at one end of the outer surface 91). In this arrangement, the point at which burrs are produced is separate from both the side face 90 and the outer periphery 91 and at the corner therebetween. Such burrs are easily removable.
But if radial through grooves 89 are formed in the seal ring to achieve the second object, that is, to form a lubricant film over the entire area of the sealing surface, such grooves will be clogged with burrs 92 when the seal ring is formed by injection molding using the abovementioned mold 92. Thus, it is impossible to achieve the second object.
A third object of this invention is to provide a seal ring of a synthetic resin which can form a lubricant film over the entire area of the sealing surface without being influenced by burrs produced when molding the seal ring.